Thrombokinase has been isolated from bovine plasma in a state approaching electrophoretic homogeneity at pH 8.6; and it has given a single boundary in the ultracentrifuge. Thrombokinase can function alone as direct activator of prothrombin; but its effect can be augmented by platelets, calcium, and factor V. It can also hydrolyze substituted arginine esters; and it is susceptible to soybean trypsin inhibitor. While it is thus a trypsin-like enzyme, it has little caseinolytic activity. As a factor in blood coagulation, thrombokinase may become one of a series of enzymes with activator functions. And these functions may not be absolutely specific. For example, thrombokinase preparations can also activate chymotrypsinogen. Test systems have been developed for quantifying the activations of chymotrypsinogen and trypsinogen by enzymes involved in blood clotting and fibrinolysis. The results have been compared with the activity of these enzymes on synthetic substrates. Continued attention is being given to the relation of thrombokinase to the factors associated with hemorrhagic diseases and to factors of importance in fibrinolysis. All data indicate that factor Xa, autoprothrombin C, and thrombokinase are equivalent, and that this represents the only prothrombin-activating enzyme of physiologic importance isolated to date. Variant thrombokinase appears slowly in aging concentrates, faster with added chymotrypsin. It will be seen whether trypsin also accelerates appearance of variant, which is less than 1/100 as susceptible to augmentation by accessory factors. Is there more than one such variant? With the aid of variant thrombokinase, the biochemical functions of phospholipid, Ca ions, and factor V will be investigated. Attempts to crystallize thrombokinase will continue. In addition to technics already in use here, electrofocusing and preparative disc electrophoresis will be tried. The main objective of 1974-75 will be to isolate a stable prothrombin which does not have a significant contamination with thrombokinase or its precursor.